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Preparation

Diaminobenzidine, which is commercially available, is prepared by reacting 3,3'-dichlorobenzidine and ammonia with a copper catalyst at high temperature and pressure, followed by acidic workup.[2]

Applications

Diaminobenzidine is oxidized by hydrogen peroxide in the presence of hemoglobin to give a dark brown color. This is used to detect fingerprints in blood.[3] Improperly prepared tissue samples may give false positives[4]
In neuroscience, this reaction is used to stain neurons which were prepared to contain hydrogen peroxidase, using complexes of injected biocytin with avidine or streptavidine, biotin, and the peroxidase.